1. Field of the Invention
The present invention relates generally to the field of integrated circuits. More particularly, the present invention relates to the field of waveform generators.
2. Discussion of Related Art
Often times it is desirable to generate waveforms having very precise rise and fall times. For example, very precise rise and fall times in communication applications such as an Ethernet application may be required to eliminate the higher frequency harmonics. In an Ethernet application, an input waveform having a very sharp rise and fall time (e.g. a rectangular wave having a nearly infinite slope) will capture the higher frequency harmonics (e.g. 30, 50, 70 Megahertzs) that are present on the signals but by slowing down the rise and fall times of the waveform (e.g. a trapezoidal waveform having a gradual slope), the higher frequency harmonics are eliminated and the lower frequency harmonics (e.g. 10 Megahertz) are captured.
Typically, waveshaping and filtering techniques are used to generate waveforms having a desired slope. In the prior art shown in FIG. 1, a Low Pass Filter 100 receives an a pulse waveform 110 at Input 101 and generates a waveform 112 at Output 102. The Low Pass Filter 100 passes signal frequencies of interest while rejecting undesired signal frequencies. Although the Low Pass Filter 100 reduces the rise and fall time of the input waveform 110, the output waveform generated does not have symmetrical rise and fall times as shown in FIG. 1. The curvature of the output waveform 112 is a result of the transient response of the Low Pass Filter 100. For certain applications, the waveform generated by the Low Pass Filter 100 does not provide a waveform having rise and fall times that are precise enough to function properly in the application.
FIG. 2a illustrates a waveform generator 200 that utilizes a comparator 201 to generate the free running triangular output signal 210 shown in FIG. 2b. Note that the signal 210 generated by the waveform generator 200 is a symmetrical waveform having rise and fall times with opposite slopes. Generally, a comparator compares the input voltage .upsilon..sub.I with a waveform voltage V.sub.W that constitutes the comparator threshold. The waveform voltage V.sub.W is generated by the feedback loop which includes the current sources 203 and 204 and the capacitor 202. If the input voltage .upsilon..sub.I is greater than the waveform voltage V.sub.W, the comparator provides a high level output at node 205. Alternatively, if the input voltage .upsilon..sub.I is less than the waveform voltage V.sub.W, the comparator provides a low level output at node 205. The voltage at node 205 is coupled to the inverting input voltage .upsilon..sub.I of the comparator 201 whose non-inverting input is coupled to node 206. The output signal 210 in FIG. 2b has a maximum amplitude of V.sub.H and a minimum amplitude of V.sub.L over a period T.
When the voltage at node 205 is at a high level, the current source 204 is enabled and the current source 203 is disabled. Also, the switch 207 has coupled .upsilon..sub.I to V.sub.L, to the low voltage reference. Typically, each of the current sources 204 and 203 generate the same amount of current when enabled. When this occurs, the current I.sub.b generated by current source 204 flows out of node 206 and out of the capacitor 202 causing the waveform voltage V.sub.W to decrease. The comparator 201 then compares the waveform voltage V.sub.W with the input voltage .upsilon..sub.I and generates an output voltage at node 205. Thus, when the waveform voltage V.sub.W decreases past the input voltage .upsilon..sub.I, the output of the comparator at 205 will change state from high to low.
When the voltage at node 205 is low, the current source 203 is enabled and the current source 204 is disabled. Additionally, the switch 207 couples .upsilon..sub.I to V.sub.H, the high voltage reference. When this occurs, the current I.sub.a generated by the current source 203 flows into node 206 and into the capacitor 202, charging it up and increasing the waveform voltage V.sub.W. The comparator 201 compares the waveform voltage V.sub.W with the input voltage .upsilon..sub.I, the output of the comparator at 205 will change state from low to high.
Although the waveform generator 200 in FIG. 2 generates a triangular output signal having symmetrical rise and fall times, the waveform generator 200 is free running. The circuit in the present invention is desirable for applications that require a waveform having symmetrical rise and fall times that operate in response to an input signal such as a rectangular pulse waveform. Furthermore, the output signal may be a trapezoidal waveform having a specified offset value and a specified amplitude.